Setupmeta

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* Test with 3.10, publish sdist with 3.9

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* Do not auto-fill ``tests_require`` any more, it is deprecated,
see https://setuptools.readthedocs.io/en/latest/userguide/dependency_management.html#optional-dependencies

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* Do not abstract ``requirements.in`` files any more, use them as-is (since they're intended to
be a spec for pip-compile_)

* Next minor version of setupmeta will not auto-fill ``tests_require`` any more.

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* Don't try and modify any git url from ``requirements.txt``, it is too much of a moving target
and the whole thing is debated: https://github.com/pypa/pip/issues/5898

* Do not auto-fill any ``dependency_links`` at all

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* Yield PEP-440_ compliant versions

* Versions yielded by ``setupmeta`` 3.0 will differ from pre-3.0 versions:

* Character ``"+"`` is used exclusively to demarcate the local part of the version

* Character ``"."`` is used exclusively to demarcate local segments
(this is not configurable yet, and won't be unless by popular request)

* The "main" part remains intact, except for ``devcommit``, furthermore only known PEP-440
"main version part" bits can ever be in the main part (anything that is mistakenly
specified to be in the main part gets automatically shifted to the local part)

* "local" part is always shown now, no need to use ``"!"`` character (now deprecated),
except for ``{devcommit}`` and ``{dirty}`` markers, which will not lead to a local part
being shown when exactly on git tag.

* See [pep-440.rst](./docs/pep-440.rst) for more details

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* Don't auto-complete ``classifiers`` from ``classifiers.txt`` any more,
the added value of that is negligible (and potentially confusing)